The Podcast Guest Guide by Antony Johnston was brought up on a podcast a while ago - I can’t remember which one. It also contains links to articles by Jason Snell, Marco Arment and Dan Benjamin on the practicalities of podcasting and Trin Garratino from Kickstarter Games with some “tips on how to sound more relaxed and natural”.

what have people found is the best way to ensure proper audio from them?

The best way is for them to have a semi decent microphone at least, and to record their own audio. While you may not need a “fancy” microphone in general the more you spend the better you sound, and as it’s the communication method with the listener it’s worth investing in making it sound good. Marco Arment has a good rundown of mics:

Also curious for a podcast that has guests on it without fancy microphone setups - what have people found is the best way to ensure proper audio from them?

I’ve been a guest on a podcast that used Zencaster to record. It records on the speaker’s end (so you get a “double ender”) and I just used the mic built into my EarPods. It doesn’t sound as good as a proper microphone setup, but I haven’t heard anyone complain about it. Didn’t even have to do anything special to upload; when the recording stops, Zencaster automatically packages it all up for the host/organizer.